I have had one of this design running all winter and we did some datalogging on it to measure the performance. I would rate it as a 400 watt machine. Sometimes the power output will surge to about twice this much but the maximum sustained/average output is about 400 watts.

You could use 15mm thick magnets yes but it would run a bit fast. You’d have to reduce the air gap to a much closer clearance. You’d probably get acceptable performance with the same stator but I’d be inclined to use more turns of thinner wire for the best output in low winds. And then the maximum output would be lower and the tail would need to be a little bit lighter too.